Rollup merge of #109410 - fmease:iat-alias-kind-inherent, r=compiler-…

…errors

Introduce `AliasKind::Inherent` for inherent associated types

Allows us to check (possibly generic) inherent associated types for well-formedness.
Type inference now also works properly.

Follow-up to #105961. Supersedes #108430.
Fixes #106722.
Fixes #108957.
Fixes #109768.
Fixes #109789.
Fixes #109790.

~Not to be merged before #108860 (`AliasKind::Weak`).~

CC `@jackh726`
r? `@compiler-errors`

`@rustbot` label T-types F-inherent_associated_types

compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -2419,6 +2419,10 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             return Ok(None);
         }
 
+        //
+        // Select applicable inherent associated type candidates modulo regions.
+        //
+
         // In contexts that have no inference context, just make a new one.
         // We do need a local variable to store it, though.
         let infcx_;
@@ -2431,14 +2435,18 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             }
         };
 
-        let param_env = tcx.param_env(block.owner.to_def_id());
+        // FIXME(inherent_associated_types): Acquiring the ParamEnv this early leads to cycle errors
+        // when inside of an ADT (#108491) or where clause.
+        let param_env = tcx.param_env(block.owner);
         let cause = ObligationCause::misc(span, block.owner.def_id);
 
         let mut fulfillment_errors = Vec::new();
         let mut applicable_candidates: Vec<_> = infcx.probe(|_| {
             let universe = infcx.create_next_universe();
 
             // Regions are not considered during selection.
+            // FIXME(non_lifetime_binders): Here we are "truncating" or "flattening" the universes
+            // of type and const binders. Is that correct in the selection phase? See also #109505.
             let self_ty = tcx.replace_escaping_bound_vars_uncached(
                 self_ty,
                 FnMutDelegate {
@@ -2454,41 +2462,40 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
 
             candidates
                 .iter()
-                .filter_map(|&(impl_, (assoc_item, def_scope))| {
+                .copied()
+                .filter(|&(impl_, _)| {
                     infcx.probe(|_| {
                         let ocx = ObligationCtxt::new_in_snapshot(&infcx);
 
-                        let impl_ty = tcx.type_of(impl_);
                         let impl_substs = infcx.fresh_item_substs(impl_);
-                        let impl_ty = impl_ty.subst(tcx, impl_substs);
+                        let impl_ty = tcx.type_of(impl_).subst(tcx, impl_substs);
                         let impl_ty = ocx.normalize(&cause, param_env, impl_ty);
 
-                        // Check that the Self-types can be related.
-                        // FIXME(fmease): Should we use `eq` here?
-                        ocx.sup(&ObligationCause::dummy(), param_env, impl_ty, self_ty).ok()?;
+                        // Check that the self types can be related.
+                        // FIXME(inherent_associated_types): Should we use `eq` here? Method probing uses
+                        // `sup` for this situtation, too. What for? To constrain inference variables?
+                        if ocx.sup(&ObligationCause::dummy(), param_env, impl_ty, self_ty).is_err()
+                        {
+                            return false;
+                        }
 
                         // Check whether the impl imposes obligations we have to worry about.
-                        let impl_bounds = tcx.predicates_of(impl_);
-                        let impl_bounds = impl_bounds.instantiate(tcx, impl_substs);
-
+                        let impl_bounds = tcx.predicates_of(impl_).instantiate(tcx, impl_substs);
                         let impl_bounds = ocx.normalize(&cause, param_env, impl_bounds);
-
                         let impl_obligations = traits::predicates_for_generics(
                             |_, _| cause.clone(),
                             param_env,
                             impl_bounds,
                         );
-
                         ocx.register_obligations(impl_obligations);
 
                         let mut errors = ocx.select_where_possible();
                         if !errors.is_empty() {
                             fulfillment_errors.append(&mut errors);
-                            return None;
+                            return false;
                         }
 
-                        // FIXME(fmease): Unsolved vars can escape this InferCtxt snapshot.
-                        Some((assoc_item, def_scope, infcx.resolve_vars_if_possible(impl_substs)))
+                        true
                     })
                 })
                 .collect()
@@ -2497,24 +2504,26 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         if applicable_candidates.len() > 1 {
             return Err(self.complain_about_ambiguous_inherent_assoc_type(
                 name,
-                applicable_candidates.into_iter().map(|(candidate, ..)| candidate).collect(),
+                applicable_candidates.into_iter().map(|(_, (candidate, _))| candidate).collect(),
                 span,
             ));
         }
 
-        if let Some((assoc_item, def_scope, impl_substs)) = applicable_candidates.pop() {
+        if let Some((impl_, (assoc_item, def_scope))) = applicable_candidates.pop() {
             self.check_assoc_ty(assoc_item, name, def_scope, block, span);
 
-            // FIXME(inherent_associated_types): To fully *confirm* the *probed* candidate, we still
-            // need to relate the Self-type with fresh item substs & register region obligations for
-            // regionck to prove/disprove.
-
-            let item_substs =
-                self.create_substs_for_associated_item(span, assoc_item, segment, impl_substs);
+            // FIXME(fmease): Currently creating throwaway `parent_substs` to please
+            // `create_substs_for_associated_item`. Modify the latter instead (or sth. similar) to
+            // not require the parent substs logic.
+            let parent_substs = InternalSubsts::identity_for_item(tcx, impl_);
+            let substs =
+                self.create_substs_for_associated_item(span, assoc_item, segment, parent_substs);
+            let substs = tcx.mk_substs_from_iter(
+                std::iter::once(ty::GenericArg::from(self_ty))
+                    .chain(substs.into_iter().skip(parent_substs.len())),
+            );
 
-            // FIXME(fmease, #106722): Check if the bounds on the parameters of the
-            // associated type hold, if any.
-            let ty = tcx.type_of(assoc_item).subst(tcx, item_substs);
+            let ty = tcx.mk_alias(ty::Inherent, tcx.mk_alias_ty(assoc_item, substs));
 
             return Ok(Some((ty, assoc_item)));
         }

compiler/rustc_hir_analysis/src/coherence/orphan.rs

@@ -210,6 +210,19 @@ fn do_orphan_check_impl<'tcx>(
                 NonlocalImpl::DisallowOther,
             ),
 
+            // ```
+            // struct S<T>(T);
+            // impl<T: ?Sized> S<T> {
+            //     type This = T;
+            // }
+            // impl<T: ?Sized> AutoTrait for S<T>::This {}
+            // ```
+            // FIXME(inherent_associated_types): The example code above currently leads to a cycle
+            ty::Alias(AliasKind::Inherent, _) => (
+                LocalImpl::Disallow { problematic_kind: "associated type" },
+                NonlocalImpl::DisallowOther,
+            ),
+
             // type Opaque = impl Trait;
             // impl AutoTrait for Opaque {}
             ty::Alias(AliasKind::Opaque, _) => (

compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs

@@ -1948,7 +1948,7 @@ fn is_late_bound_map(
                 ty::Param(param_ty) => {
                     self.arg_is_constrained[param_ty.index as usize] = true;
                 }
-                ty::Alias(ty::Projection, _) => return ControlFlow::Continue(()),
+                ty::Alias(ty::Projection | ty::Inherent, _) => return ControlFlow::Continue(()),
                 _ => (),
             }
             t.super_visit_with(self)

compiler/rustc_hir_analysis/src/collect/type_of.rs

@@ -127,7 +127,7 @@ fn anon_const_type_of<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Ty<'tcx> {
             // the def_id that this query was called with. We filter to only type and const args here
             // as a precaution for if it's ever allowed to elide lifetimes in GAT's. It currently isn't
             // but it can't hurt to be safe ^^
-            if let ty::Alias(ty::Projection, projection) = ty.kind() {
+            if let ty::Alias(ty::Projection | ty::Inherent, projection) = ty.kind() {
                 let generics = tcx.generics_of(projection.def_id);
 
                 let arg_index = segment

compiler/rustc_hir_analysis/src/constrained_generic_params.rs

@@ -59,7 +59,7 @@ struct ParameterCollector {
 impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for ParameterCollector {
     fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
         match *t.kind() {
-            ty::Alias(ty::Projection, ..) if !self.include_nonconstraining => {
+            ty::Alias(ty::Projection | ty::Inherent, ..) if !self.include_nonconstraining => {
                 // projections are not injective
                 return ControlFlow::Continue(());
             }

compiler/rustc_hir_analysis/src/outlives/implicit_infer.rs

@@ -210,6 +210,9 @@ fn insert_required_predicates_to_be_wf<'tcx>(
                 );
             }
 
+            // FIXME(inherent_associated_types): Handle this case properly.
+            ty::Alias(ty::Inherent, _) => {}
+
             _ => {}
         }
     }

compiler/rustc_hir_typeck/src/fn_ctxt/adjust_fulfillment_errors.rs

@@ -843,7 +843,7 @@ fn find_param_in_ty<'tcx>(
             return true;
         }
         if let ty::GenericArgKind::Type(ty) = arg.unpack()
-                && let ty::Alias(ty::Projection, ..) = ty.kind()
+                && let ty::Alias(ty::Projection | ty::Inherent, ..) = ty.kind()
             {
                 // This logic may seem a bit strange, but typically when
                 // we have a projection type in a function signature, the

compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs

@@ -300,7 +300,7 @@ impl<'a, 'tcx> AstConv<'tcx> for FnCtxt<'a, 'tcx> {
         match ty.kind() {
             ty::Adt(adt_def, _) => Some(*adt_def),
             // FIXME(#104767): Should we handle bound regions here?
-            ty::Alias(ty::Projection, _) if !ty.has_escaping_bound_vars() => {
+            ty::Alias(ty::Projection | ty::Inherent, _) if !ty.has_escaping_bound_vars() => {
                 self.normalize(span, ty).ty_adt_def()
             }
             _ => None,

compiler/rustc_hir_typeck/src/method/suggest.rs

@@ -2211,7 +2211,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         | ty::Float(_)
                         | ty::Adt(_, _)
                         | ty::Str
-                        | ty::Alias(ty::Projection, _)
+                        | ty::Alias(ty::Projection | ty::Inherent, _)
                         | ty::Param(_) => format!("{deref_ty}"),
                         // we need to test something like  <&[_]>::len or <(&[u32])>::len
                         // and Vec::function();

compiler/rustc_infer/src/infer/combine.rs

@@ -127,7 +127,8 @@ impl<'tcx> InferCtxt<'tcx> {
                 bug!()
             }
 
-            (_, ty::Alias(AliasKind::Projection, _)) | (ty::Alias(AliasKind::Projection, _), _)
+            (_, ty::Alias(AliasKind::Projection | AliasKind::Inherent, _))
+            | (ty::Alias(AliasKind::Projection | AliasKind::Inherent, _), _)
                 if self.tcx.trait_solver_next() =>
             {
                 relation.register_type_relate_obligation(a, b);

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -2354,7 +2354,9 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         let labeled_user_string = match bound_kind {
             GenericKind::Param(ref p) => format!("the parameter type `{}`", p),
             GenericKind::Alias(ref p) => match p.kind(self.tcx) {
-                ty::AliasKind::Projection => format!("the associated type `{}`", p),
+                ty::AliasKind::Projection | ty::AliasKind::Inherent => {
+                    format!("the associated type `{}`", p)
+                }
                 ty::AliasKind::Opaque => format!("the opaque type `{}`", p),
             },
         };

compiler/rustc_infer/src/infer/error_reporting/note_and_explain.rs

@@ -71,9 +71,10 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                              #traits-as-parameters",
                         );
                     }
-                    (ty::Alias(ty::Projection, _), ty::Alias(ty::Projection, _)) => {
+                    (ty::Alias(ty::Projection | ty::Inherent, _), ty::Alias(ty::Projection | ty::Inherent, _)) => {
                         diag.note("an associated type was expected, but a different one was found");
                     }
+                    // FIXME(inherent_associated_types): Extend this to support `ty::Inherent`, too.
                     (ty::Param(p), ty::Alias(ty::Projection, proj)) | (ty::Alias(ty::Projection, proj), ty::Param(p))
                         if !tcx.is_impl_trait_in_trait(proj.def_id) =>
                     {
@@ -222,7 +223,7 @@ impl<T> Trait<T> for X {
                             diag.span_label(p_span, "this type parameter");
                         }
                     }
-                    (ty::Alias(ty::Projection, proj_ty), _) if !tcx.is_impl_trait_in_trait(proj_ty.def_id) => {
+                    (ty::Alias(ty::Projection | ty::Inherent, proj_ty), _) if !tcx.is_impl_trait_in_trait(proj_ty.def_id) => {
                         self.expected_projection(
                             diag,
                             proj_ty,
@@ -231,7 +232,7 @@ impl<T> Trait<T> for X {
                             cause.code(),
                         );
                     }
-                    (_, ty::Alias(ty::Projection, proj_ty)) if !tcx.is_impl_trait_in_trait(proj_ty.def_id) => {
+                    (_, ty::Alias(ty::Projection | ty::Inherent, proj_ty)) if !tcx.is_impl_trait_in_trait(proj_ty.def_id) => {
                         let msg = format!(
                             "consider constraining the associated type `{}` to `{}`",
                             values.found, values.expected,

compiler/rustc_infer/src/infer/opaque_types.rs

@@ -549,6 +549,7 @@ impl<'tcx> InferCtxt<'tcx> {
                     // We can't normalize associated types from `rustc_infer`,
                     // but we can eagerly register inference variables for them.
                     // FIXME(RPITIT): Don't replace RPITITs with inference vars.
+                    // FIXME(inherent_associated_types): Extend this to support `ty::Inherent`, too.
                     ty::Alias(ty::Projection, projection_ty)
                         if !projection_ty.has_escaping_bound_vars()
                             && !tcx.is_impl_trait_in_trait(projection_ty.def_id) =>
@@ -569,6 +570,7 @@ impl<'tcx> InferCtxt<'tcx> {
                         hidden_ty
                     }
                     // FIXME(RPITIT): This can go away when we move to associated types
+                    // FIXME(inherent_associated_types): Extend this to support `ty::Inherent`, too.
                     ty::Alias(
                         ty::Projection,
                         ty::AliasTy { def_id: def_id2, substs: substs2, .. },

compiler/rustc_lint/src/builtin.rs

@@ -62,6 +62,7 @@ use rustc_middle::lint::in_external_macro;
 use rustc_middle::ty::layout::{LayoutError, LayoutOf};
 use rustc_middle::ty::print::with_no_trimmed_paths;
 use rustc_middle::ty::subst::GenericArgKind;
+use rustc_middle::ty::TypeVisitableExt;
 use rustc_middle::ty::{self, Instance, Ty, TyCtxt, VariantDef};
 use rustc_session::config::ExpectedValues;
 use rustc_session::lint::{BuiltinLintDiagnostics, FutureIncompatibilityReason};
@@ -1442,6 +1443,10 @@ impl<'tcx> LateLintPass<'tcx> for TypeAliasBounds {
             // Bounds are respected for `type X = impl Trait`
             return;
         }
+        if cx.tcx.type_of(item.owner_id).skip_binder().has_inherent_projections() {
+            // Bounds are respected for `type X = … Type::Inherent …`
+            return;
+        }
         // There must not be a where clause
         if type_alias_generics.predicates.is_empty() {
             return;
@@ -1561,7 +1566,6 @@ declare_lint_pass!(
 
 impl<'tcx> LateLintPass<'tcx> for TrivialConstraints {
     fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::Item<'tcx>) {
-        use rustc_middle::ty::visit::TypeVisitableExt;
         use rustc_middle::ty::Clause;
         use rustc_middle::ty::PredicateKind::*;
 
@@ -2898,6 +2902,7 @@ impl ClashingExternDeclarations {
                         | (Generator(..), Generator(..))
                         | (GeneratorWitness(..), GeneratorWitness(..))
                         | (Alias(ty::Projection, ..), Alias(ty::Projection, ..))
+                        | (Alias(ty::Inherent, ..), Alias(ty::Inherent, ..))
                         | (Alias(ty::Opaque, ..), Alias(ty::Opaque, ..)) => false,
 
                         // These definitely should have been caught above.

compiler/rustc_lint/src/types.rs

@@ -1119,14 +1119,14 @@ impl<'a, 'tcx> ImproperCTypesVisitor<'a, 'tcx> {
 
             // `extern "C" fn` functions can have type parameters, which may or may not be FFI-safe,
             //  so they are currently ignored for the purposes of this lint.
-            ty::Param(..) | ty::Alias(ty::Projection, ..)
+            ty::Param(..) | ty::Alias(ty::Projection | ty::Inherent, ..)
                 if matches!(self.mode, CItemKind::Definition) =>
             {
                 FfiSafe
             }
 
             ty::Param(..)
-            | ty::Alias(ty::Projection, ..)
+            | ty::Alias(ty::Projection | ty::Inherent, ..)
             | ty::Infer(..)
             | ty::Bound(..)
             | ty::Error(_)

compiler/rustc_middle/src/query/mod.rs

@@ -1821,6 +1821,16 @@ rustc_queries! {
         desc { "normalizing `{}`", goal.value.value }
     }
 
+    /// Do not call this query directly: invoke `normalize` instead.
+    query normalize_inherent_projection_ty(
+        goal: CanonicalProjectionGoal<'tcx>
+    ) -> Result<
+        &'tcx Canonical<'tcx, canonical::QueryResponse<'tcx, NormalizationResult<'tcx>>>,
+        NoSolution,
+    > {
+        desc { "normalizing `{}`", goal.value.value }
+    }
+
     /// Do not call this query directly: invoke `try_normalize_erasing_regions` instead.
     query try_normalize_generic_arg_after_erasing_regions(
         goal: ParamEnvAnd<'tcx, GenericArg<'tcx>>

compiler/rustc_middle/src/ty/context.rs

@@ -1848,7 +1848,17 @@ impl<'tcx> TyCtxt<'tcx> {
         let substs = substs.into_iter().map(Into::into);
         #[cfg(debug_assertions)]
         {
-            let n = self.generics_of(_def_id).count();
+            let generics = self.generics_of(_def_id);
+
+            let n = if let DefKind::AssocTy = self.def_kind(_def_id)
+                && let DefKind::Impl { of_trait: false } = self.def_kind(self.parent(_def_id))
+            {
+                // If this is an inherent projection.
+
+                generics.params.len() + 1
+            } else {
+                generics.count()
+            };
             assert_eq!(
                 (n, Some(n)),
                 substs.size_hint(),
@@ -2009,7 +2019,7 @@ impl<'tcx> TyCtxt<'tcx> {
         debug_assert_matches!(
             (kind, self.def_kind(alias_ty.def_id)),
             (ty::Opaque, DefKind::OpaqueTy)
-                | (ty::Projection, DefKind::AssocTy)
+                | (ty::Projection | ty::Inherent, DefKind::AssocTy)
                 | (ty::Opaque | ty::Projection, DefKind::ImplTraitPlaceholder)
         );
         self.mk_ty_from_kind(Alias(kind, alias_ty))

compiler/rustc_middle/src/ty/error.rs

@@ -265,7 +265,7 @@ impl<'tcx> Ty<'tcx> {
             ty::Infer(ty::FreshTy(_)) => "fresh type".into(),
             ty::Infer(ty::FreshIntTy(_)) => "fresh integral type".into(),
             ty::Infer(ty::FreshFloatTy(_)) => "fresh floating-point type".into(),
-            ty::Alias(ty::Projection, _) => "associated type".into(),
+            ty::Alias(ty::Projection | ty::Inherent, _) => "associated type".into(),
             ty::Param(p) => format!("type parameter `{p}`").into(),
             ty::Alias(ty::Opaque, ..) => if tcx.ty_is_opaque_future(self) { "future".into() } else { "opaque type".into() },
             ty::Error(_) => "type error".into(),
@@ -312,7 +312,7 @@ impl<'tcx> Ty<'tcx> {
             ty::Tuple(..) => "tuple".into(),
             ty::Placeholder(..) => "higher-ranked type".into(),
             ty::Bound(..) => "bound type variable".into(),
-            ty::Alias(ty::Projection, _) => "associated type".into(),
+            ty::Alias(ty::Projection | ty::Inherent, _) => "associated type".into(),
             ty::Param(_) => "type parameter".into(),
             ty::Alias(ty::Opaque, ..) => "opaque type".into(),
         }